Cross aryl electrophile

Table 5.4 Reactivity profile of aryl electrophiles in iron-catalyzed cross-coupling reactions.

Since the initial report from Migita [77,78] Pd-catalyzed cross-coupling to generate aryl and alkyl sulfides has been studied extensively [14, 15, 17, 18, 28, 77-111]. The large majority of this work is dedicated to developing synthetically useful protocols for the coupling of various thiols with an appropriate vinyl or aryl electrophile. 

Terminal alkynes may be deprotonated with LDA and transmet-allated with zinc to undergo palladium-catalyzed cross-couplings with aryl electrophiles (Scheme O... 

TABLE 2. Pd-Catalyzed Cross-Coupling of Vinylmetals with Aryl Electrophiles ... 

Alkenylsilicon compounds themselves are generally inert in Pd-catalyzed cross-coupling with aryl electrophiles. However, their reactivity could be remarkably enhanced by the addition of fluorides to generate pentacoordinate alkenylsilicon derivatives that can undergo the desired cross-coupling. Some typical examples are shown in... 

Some representative examples of the cross-coupling of three different types of trisubstituted alkenylmetals with aryl electrophiles are shown in Table 6 and arranged in the order /3,/3 -, cis-a, -, and lra i-Q ,j8-substituted alkenylmetals. 

B.iv.a. p, P -Substituted Alkenybnetals. Carbometallation reactions of 1-alkynes, most notably carboalumination and carbocupration,t collectively provide a powerful route to the synthesis of /3,j8 -substituted alkenylmetals. The alkenylmetals generated in situ may be used directly for Pd- or Ni-catalyzed cross-coupling with aryl electrophiles (Protocol 1). One prototypical example of the carbometallation-cross-coupling tandem process is shown in Scheme... 

There are six discrete combinations for Pd-catalyzed cross-coupling between homoallyl-, homopropargyl-, or homobenzyhnetals and alkenyl or aryl electrophiles (Scheme 4). In addition to these processes, those involving alkyl, allyl, benzyl, propargyl, alkynyl, and acyl electrophiles are conceivable, and some have indeed been observed. However, these reactions are discussed in other pertinent sections. 

All o-hetero-substituted arylmetals and aryl electrophiles (1) as well as heteroaromatic compounds represented by 2 and 3 are /3-hetero-substituted aryl derivatives. In most cases, /3-heteroatoms merely exert some rate-enhancing or -retarding influences on the Pd-catalyzed cross-coupling process. In some cases, however, their presence offers additional synthetic opportunities of potential significance. Synthesis of arene-fused heterocyclic compounds is particularly noteworthy, as indicated by the results presented below. However, most of the known examples involve cross-coupling-heteropalladation tandem processes, which are also discussed in Sect. V.3. So, they are very briefly mentioned in this section. 

In addition to the aryl halides and sulfonates that have been used most commonly in Pd-catalyzed cross-coupling and that have been discussed above for amination, diphenyliodonium salts have been reported as aryl electrophiles for amination chemistry (Eq. 31)J Perhaps most significant, the use of diphenyliodonium salts as electrophile led to room-temperature amination chemistry using the original P(o-tolyl)3 ligand systan, as well as with Ph3As. Only chemistry with secondary amines was reported. 

CROSS-COUPLINGS OF ARYLMETALLICS WITH ARYL ELECTROPHILES... 

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